1. Field of the Invention
The present invention relates to expandable tubulars. More particularly, the invention relates to improved apparatus and methods for expanding tubular strings, including tubulars and the connections therebetween. More particularly still, the invention relates to improved apparatus and methods for expanding tubular strings through the use of expansion tools having optimized, shaped surfaces that reduce axial bending forces and damage to threaded connections.
2. Description of the Related Art
Strings of wellbore tubulars are used to line wellbores and to provide a fluid conduit for the collection of hydrocarbons. Typically, a portion of wellbore is formed by drilling and then a string of tubulars (or “liner” or “casing”), is inserted and cemented into the wellbore to prevent cave-in and to isolate the wellbore from a surrounding formation. Because the wellbore is drilled in sections and each section is cased before continuing to drill, each subsequent section is of a smaller diameter than the one above it, resulting in a telescopic arrangement of casing having an ever-decreasing diameter.
Expanding tubulars in a wellbore involves running a string of tubulars in at a first, smaller diameter and then enlarging their diameter once they are set in place. Downhole expansion has always been appealing as a way to partially overcome the limitations brought about by small diameter tubulars. For example, expanding a downhole tubular even slightly results in an enlarged fluid pathway for hydrocarbons and an enlarged pathway for the passage of a subsequent string of tubulars or tools needed for operations downhole. In another example, expandable tubulars can permit troublesome zones in a wellbore to be sealed off by running a section of tubulars into the wellbore and expanding it against the wellbore walls to isolate a formation. In still another example, expandable production tubing could be inserted into a wellbore at a first diameter and then expanded to permit greater capacity for collecting hydrocarbons.
A typical prior art expansion tool is illustrated in U.S. Pat. No. 5,348,095 and that patent is incorporated by reference herein in its entirety. The '095 patent teaches a tool having a conically shaped first end permitting its insertion into a tubular. The mid portion of the tool has an outer diameter substantially larger than the inner diameter of the tubular to be expanded. Through either fluid or mechanical force or a combination thereof, the tool is forced through the tubular, resulting in an increase in the inner and outer diameters of the tubular.
Other prior art patents illustrate techniques for moving an expansion tool through a string of tubulars. For example, U.S. Pat. No. 6,085,838, incorporated herein by reference, illustrates running a section of casing or liner into a wellbore on a work string that includes a conical expansion tool at its lower end. After the section of liner is located in the wellbore and anchored, the work string and expansion tool are moved upwards due to fluid pressure pumped through the work string and acting upon a lower end of the tool. After expanding the length of tubular, the string and expansion tool are removed, leaving the expanded liner in the wellbore.
When a tubular is expanded by moving an expansion tool through it, a frictional force is developed between the contact surface(s) of the tool and the tubular walls in contact with the tool. A radial expansion force is also created as the tubular walls move directly outwards from the centerline of the tubular. Additionally, there is a force developed along the longitudinal axis of the tubular due to the movement of the expansion tool along its length. This “axial bending” force causes the tubing to bend outwards, or flare as the tool “opens” the tubular to a greater diameter. Of the various forces at work during expansion by an expansion tool, axial bending is the most troublesome due to its progressive nature and its tendency to place an inside wall of a tubular into tension and an outer wall into compression as the cone moves along in the expansion process.
FIG. 1 is a graph showing the contact force generated by a prior art, conical expansion tool as it moves through and expands a 5½″ diameter section of tubing. The horizontal axis of the graph is the tool's expansion surface measured in inches and the vertical axis is contact pressure between the tool and tubular measured in thousands of pounds per square inch (ksi). The prior art expansion tool has a cone angle of 10 degrees and its frustoconical expansion surface is a relatively short 2″. Evident in the graph are two large spikes 101, 102 of contact force. The first spike 101 (exceeding 100 ksi) comes about due to the relatively abrupt meeting of the tool and the tubular and the second 102 results from a termination of the expansion process where the tubular extends over the trailing end of the tool. The inventors have determined that axial bending stresses are the greatest at locations where contact pressures are the highest, especially when those contact pressures are followed by relatively low pressures. In the graph of FIG. 1, the high spikes of contact pressure 101, 102 are adjacent to other areas of pressure 103, 104 so low that the tool is not even in contact with the walls of the tubular.
Axial bending stress developed by the type of tool used to produce the graph of FIG. 1 are especially damaging to connections between expandable tubulars that are expanded as the expansion tool is moved through a tubular string. FIG. 2 illustrates a typical threaded connection 150 between tubulars, like liner or casing (not shown). The connection includes a pin member 152 formed at a threaded section of the first tubular and a box member 154 formed at a threaded section of the second tubular. The threaded sections of the pin member and the box member are tapered and are formed directly into the ends of the tubular. The pin member 152 includes helical threads 153 extending along its length and terminates in a relatively thin “pin nose” portion 158. The box member 154 includes helical threads 155 that are shaped and sized to mate with the helical threads 153 of the pin member during the make-up of the threaded connection 150. The threaded section of the pin member and the box member form a connection of a predetermined integrity intended to provide not only a mechanical connection but rigidity and fluid sealing. For example, at each end of the connection, a non-threaded portion of each piece forms a metal-to-metal seal 156, 157.
Threaded connections between expandable tubulars are difficult to successfully expand because of the axial bending that takes place as an expansion member moves through the connection. For example, when a pin portion of a connector with outwardly facing threads is connected to a corresponding box portion of the connection having inwardly facing threads, the threads experience opposing forces during expansion. Typically, the outwardly facing threads will be in compression while the inwardly facing threads will be in tension. Thereafter, as the largest diameter portion of a conical expander tool moves through the connection, the forces are reversed, with the outwardly facing threads placed into tension and the inwardly facing threads in compression. The result is often a threaded connection that is loosened due to different forces acting upon the parts during expansion. Another problem relates to “spring back” that can cause a return movement of the relatively thin pin nose. Typically, threaded connections on expandable strings are placed in a wellbore in a “pin up” orientation and then expanded from the bottom upwards towards the surface. In this manner, the pin nose is the last part of the connection to be expanded. In FIG. 2 for example, the connection would be expanded from left to right.
FIG. 3 shows the threaded connection 150 of FIG. 2 after expansion with a conical expansion tool like the one shown in the '095 patent. The threads 153, 155, especially those at each end of the connection, are deformed and no longer fit tightly. The sealing areas 156, 157 are also distorted to a point where there is no longer a metal-to-metal seal formed between the parts. Damage to the threads (and sealing surfaces) is especially pronounced at each end due to the differences in thickness of the connection members towards the end of the connection. In addition to thread damage, the two portions of the connection have shifted axially at a torque shoulder, preventing the connection from remaining tightly connected and resulting in a “thinning” of a cross sectional area of the pin. Visible also is the spring back effect that has caused the pin nose portion 158 of the connection to move towards the center of the tubular. In addition to damaging a connection's sealing ability, the connection of FIG. 3 is so badly damaged it might no longer be able to resist forces tending to loosen or un-tighten the connection between the tubular members.
While the connection of FIGS. 2 and 3 show a single set of threads between the two tubulars, many expandable connections include a “two-step” thread body with threads of different diameters and little or no taper. While not illustrated, these types of connections suffer from the same problems as those with single threads when expanded by a conical shaped expander tool.
The foregoing problems with expandable tubulars and in particular, expandable connections between tubulars have been addressed by a number of prior art patents. U.S. Pat. No. 6,622,797 for instance, addresses the problem with an expansion tool having discrete segments along its profile, each segment divided by a smaller, radiused segment and resulting in an increase in diameter of the expansion tool. According to the inventors, the discrete portions create separate, discrete locations of contact between the expansion tool and the inner surface of the tubular, resulting in less friction generation and a more efficiently operating expansion process. In fact, separating the contact points necessarily creates spikes in contact forces between the tool and the tubular which can exacerbate problems associated with axial bending. In another exemplary prior art arrangement shown in U.S. Pat. No. 7,191,841, a fluid pathway is provided in the expansion tool in order to increase or decrease the force needed to move the tool through the tubular. While the forces might be adjustable, the patent drawings illustrate that the tubular walls literally “skip” off the surface of the expansion tool, creating spikes of contact pressure as the tool moves.
There is a need therefore, for an expansion tool that can expand a tubular string in a manner that decreases the likelihood of damage due to forces created during the expansion process. There is a further need for an expansion tool that can reduce contact pressures and spikes in contact pressure between the tool and the tubular or connection being expanded. There is a further need for an expansion tool that has a contact surface that can maintain contact with a tubular or connection wall and thus reduce the effects of axial bending.